physiological functions of the thyroid hormone.pptx

physiological functions of the thyroid hormone

1. PHYSIOLOGICAL FUNCTIONS OF THE THYROID HORMONES PREPARED BY FATIMA SUNDUS

2. THYROID HORMONE INCREASE TRANSCRIPTION OF MANY GENES The general effect of thyroid hormone is to activate nuclear transcription of large numbers of genes Most of the Thyroxine Secreted by the Thyroid Is Converted to Triiodothyronine . Intracellular thyroid hormone receptors have a high affinity for triiodothyronine . Consequently, more than 90 percent of the thyroid hormone molecules that bind with the receptors is triiodothyronine .

3. Thyroid Hormones Activate Nuclear Receptors . The thyroid hormone receptors are either attached to the DNA genetic strands The thyroid hormone receptor usually forms a heterodimer with retinoid X receptor (RXR ) at specific thyroid hormone response elements on the DNA. After binding with thyroid hormone, the receptors become activated and initiate the transcription process . Large numbers of different types of messenger RNA are then formed, followed within another few minutes or hours by RNA translation on the cytoplasmic ribosomes to form hundreds of new intracellular proteins .

4. Thyroid hormones also appear to have nongenomic cellular effects For example, some effects of thyroid hormones occur within minutes, too rapidly to be explained by changes in protein synthesis, and are not affected by inhibitors of gene transcription and translation . Nongenomic actions of thyroid hormone include the regulation of ion channels and oxidative phosphorylation and appear to involve the activation of intracellular secondary messengers such as cyclic adenosine monophosphate ( cAMP ) or protein kinase signaling cascades

5. THYROID HORMONES INCREASE CELLULAR METABOLIC ACTIVITY The thyroid hormones increase the metabolic activities of almost all the tissues of the body. The basal metabolic rate can increase to 60 to 100 percent above normal when large quantities of the hormones are secreted . The rate of utilization of foods for energy is greatly accelerated . Although the rate of protein synthesis is increased , at the same time the rate of protein catabolism is also increased . The growth rate of young people is greatly accelerated . The mental processes are excited, and the activities of most of the other endocrine glands are increased .

6. Thyroid Hormones Increase the Number and Activity of Mitochondria. When thyroxine or triiodothyronine is given to an animal, the mitochondria in most cells of the animal’s body increase in size and number . Therefore , one of the principal functions of thyroxine might be simply to increase the number and activity of mitochondria , which in turn increases the rate of formation of adenosine triphosphate to energize cellular function . However, the increase in the number and activity of mitochondria could be the result of increased activity of the cells

7. Thyroid Hormones Increase Active Transport of Ions Through Cell Membranes. One of the enzymes that increases its activity in response to thyroid hormone is Na-K-ATPase . This increased activity in turn increases the rate of transport of both sodium and potassium ions through the cell membranes of some tissues . Because this process uses energy and increases the amount of heat produced in the body, it has been suggested that this might be one of the mechanisms by which thyroid hormone increases the body’s metabolic rate . In fact, thyroid hormone also causes the cell membranes of most cells to become leaky to sodium ions , which further activates the sodium pump and further increases heat production .

8. EFFECT OF THYROID HORMONE ON GROWTH thyroid hormone is essential for the metamorphic change of the tadpole into the frog . In children with hypothyroidism , the rate of growth is greatly retarded . In children with hyperthyroidism , excessive skeletal growth often occurs, causing the child to become considerably taller at an earlier age .

9. An important effect of thyroid hormone is to promote growth and development of the brain during fetal life If the fetus does not secrete sufficient quantities of thyroid hormone, growth and maturation of the brain both before birth and afterward are greatly retarded and the brain remains smaller than normal . Without specific thyroid therapy within days or weeks after birth, the child without a thyroid gland will remain mentally deficient throughout life .

10. EFFECTS OF THYROID HORMONE ON SPECIFIC BODY FUNCTIONS Stimulation of Carbohydrate Metabolism . Thyroid hormone stimulates almost all aspects of carbohydrate metabolism , including rapid glucose uptake by cells, enhanced glycolysis , enhanced gluconeogenesis , increased rate of absorption from the gastrointestinal tract , and even increased insulin secretion with its resultant secondary effects on carbohydrate metabolism.

11. Stimulation of Fat Metabolism. Mobilization of lipids from fat tissue also increases the free fatty acid concentration in the plasma and greatly accelerates the oxidation of free fatty acids by the cells.

12. Effect on Plasma and Liver Fats . Increased thyroid hormone decreases the concentrations of cholesterol, phospholipids , and triglycerides in the plasma , even though it increases the free fatty acids . Conversely, decreased thyroid secretion greatly increases the plasma concentrations of cholesterol, phospholipids, and triglycerides and almost always causes excessive deposition of fat in the liver as well . One of the mechanisms by which thyroid hormone decreases plasma cholesterol concentration is to increase significantly cholesterol secretion in the bile and consequent loss in the feces

13. A possible mechanism for the increased cholesterol secretion is that thyroid hormone induces increased numbers of low-density lipoprotein receptors on the liver cells , leading to rapid removal of low-density lipoproteins from the plasma by the liver and subsequent secretion of cholesterol in these lipoproteins by the liver cells.

14. Increased Requirement for Vitamins. Because thyroid hormone increases the quantities of many bodily enzymes and because vitamins are essential parts of some of the enzymes or coenzymes, thyroid hormone increases the need for vitamins. Therefore , a relative vitamin deficiency can occur when excess thyroid hormone is secreted, unless at the same time increased quantities of vitamins are made available .

15. Increased Basal Metabolic Rate. Because thyroid hormone increases metabolism in almost all cells of the body , excessive quantities of the hormone can occasionally increase the basal metabolic rate 60 to 100 percent above normal. Conversely, when no thyroid hormone is produced, the basal metabolic rate falls to almost one-half normal

16. Decreased Body Weight . A greatly increased amount of thyroid hormone almost always decreases body weight a greatly decreased amount of thyroid hormone almost always increases body weight however , these effects do not always occur because thyroid hormone also increases the appetite , which may counterbalance the change in the metabolic rate.

17. Increased Blood Flow and Cardiac Output . Increased metabolism in the tissues causes more rapid utilization of oxygen than norma l These effects cause vasodilation in most body tissues, thus increasing blood flow. The rate of blood flow in the skin especially increases because of the increased need for heat elimination from the body . As a consequence of the increased blood flow, cardiac output also increases, sometimes rising to 60 percent or more above normal

18. Increased Heart Rate . The heart rate increases considerably more under the influence of thyroid hormone than would be expected from the increase in cardiac output . Therefore, thyroid hormone seems to have a direct effect on the excitability of the heart, which in turn increases the heart rate.

19. Increased Heart Strength . This effect is analogous to the increase in heart strength that occurs in mild fevers and during exercise. Indeed , some severely thyrotoxic patients die of cardiac decompensation secondary to myocardial failure and increased cardiac load imposed by the increase in cardiac output .

20. Normal Arterial Pressure The mean arterial pressure usually remains about norma l after administration of thyroid hormone . Because of increased blood flow through the tissues between heartbeats, the pulse pressure is often increased

21. Increased Respiration. The increased rate of metabolism increases the utilization of oxygen and the formation of carbon dioxide; these effects activate all the mechanisms that increase the rate and depth of respiration . Increased Gastrointestinal Motility. In addition to increased appetite and food intake, which has been discussed, thyroid hormone increases both the rates of secretion of the digestive juices and the motility of the gastrointestinal tract . Hyperthyroidism therefore often results in diarrhea, whereas lack of thyroid hormone can cause constipation .

22. Excitatory Effects on the Central Nervous System. thyroid hormone increases the rapidity of cerebration lack of thyroid hormone decreases rapidity of cerebration . A person with hyperthyroidism is likely to be extremely nervous and have many psychoneurotic tendencies , such as anxiety complexes, extreme worry, and paranoia .

23. Effect on the Function of the Muscles. A slight increase in thyroid hormone usually makes the muscles react with vigor but when the quantity of hormone becomes excessive, the muscles become weakened because of excess protein catabolism . Conversely, lack of thyroid hormone causes the muscles to become sluggish, and they relax slowly after a contraction .

24. Muscle Tremor. One of the most characteristic signs of hyperthyroidism is a fine muscle tremor. This symptom is not the coarse tremor that occurs in Parkinson’s disease or when a person shivers because it occurs at the rapid frequency of 10 to 15 times per second. The tremor can be observed easily by placing a sheet of paper on the extended fingers and noting the degree of vibration of the paper

25. Effect on Sleep . Because of the exhausting effect of thyroid hormone on the musculature and on the central nervous system, persons with hyperthyroidism often have a feeling of constant tiredness , but because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep .

26. Effect on Other Endocrine Glands. Increased thyroid hormone increases the rates of secretion of several other endocrine glands, but it also increases the need of the tissues for the hormones. For instance, increased thyroxine secretion increases the rate of glucose metabolism almost everywhere in the body and therefore causes a corresponding need for increased insulin secretion by the pancreas.

27. Also , thyroid hormone increases many metabolic activities related to bone formation and, as a consequence, increases the need for parathyroid hormone . Thyroid hormone also increases the rate at which adrenal glucocorticoid s are inactivated by the liver . This increased rate of inactivation leads to feedback increase in adrenocorticotropic hormone production by the anterior pituitary and , therefore, an increased rate of glucocorticoid secretion by the adrenal glands.

28. Effect of Thyroid Hormone on Sexual Function. In men, lack of thyroid hormone is likely to cause loss of libido ; a great excess of the hormone , however, sometimes causes impotence . In women, lack of thyroid hormone often causes menorrhagia and polymenorrhea —that is, excessive and frequent menstrual bleeding, respectively . Yet , strangely enough , in other women a lack of thyroid hormone may cause irregular periods and occasionally even amenorrhea (absence of menstrual bleeding ). Hypothyroidism in women, as in men, is likely to result in a greatly decreased libido.

29. To make the picture still more confusing, in women with hyperthyroidism, oligomenorrhea (greatly reduced bleeding) is common, and occasionally amenorrhea occurs .